If you're going to be making periodic top-ups you obviously don't want a card that charges you for doing so. Similarly (because you'll be keeping the card for quite some time rather than just loading immediately it before you travel) you don't want a card that charges a monthly fee. So the Post Office Travel Money Card (in Euros) might well suit you. The exchange rate currently offered in
marginally below that offered by some other card providers but you don't pay a fee to acquire the card, there are no monthly charges and there are no top-up charges (although you do have to top-up with a minimum of £50 on each occasion that you do so). Further, at just €2, the charge for withdrawing cash from an ATM is quite reasonable (as long as you take out relatively large amounts each time - a €200 withdrawal would cost you €2 but taking out the same amount in €50 instalments would cost you a total of €8).
http://www.postoffice.co.uk/travel-money/card
As I've indicated above, there are cards with somewhat better exchange rates available (giving you up to an extra €25 for each £1000 you put onto the card) but you could easily end up worse of with some of them through other charges.
Normally though it makes far more sense just to use a standard debit card abroad but I accept that it's far easier to get hold of a prepaid card than it is to open a separate bank account to get a debit card which can just be used for your holidays. (Written a day after a half-hour interview with Santander just to try to switch a savings account to a current account - and I still won't know until next week whether I've been successful!)